1. Field of the Invention
The present invention relates to a focus detection technique used in an image capturing apparatus such as an electronic camera.
2. Description of the Related Art
A conventional focus detection apparatus adopting a pupil division method using an image sensor is disclosed in, for example, Japanese Patent Laid-Open No. 01-216306. Such a focus detection apparatus adopting a pupil division method can detect a defocus amount by performing measurement only once even in a greatly defocused state. This method is promising because high-speed focus adjustment is possible.
On the other hand, applying differential filtering to an image signal is generally performed as a technique to improve the accuracy of defocus amount detection. An example of the effect of the differential filtering is an improvement in the direction detection accuracy. Japanese Patent Laid-Open No. 07-318793 discloses performing two types of differential filtering to improve defocus detection accuracy when vignetting has occurred.
In an apparatus that divides the pupil by dividing a photoelectric conversion unit, however, the pupil division characteristic near the optical axis is sometimes insufficient. In addition, when the f-number is small, the asymmetry of an image signal is large. For this reason, distortion is enhanced by differential filtering, which may cause a difficulty in even detecting the direction of defocus.